1. Field of the Invention
The present invention relates generally to silicon based thin film deposition for use in photovoltaic (PV) devices and methods of making the same. More particularly, the invention relates to improved methods for increasing the efficiency of the deposition of silicon based thin films on photovoltaic substrates.
2. Discussion of the Background
All United States and Foreign Patents and Published Patent Applications referred to herein are hereby incorporated by reference in their entireties. In the case of conflict, the present specification, including definitions, will control.
Of the alternative sources, the sun is considered the most abundant natural resource, with an infinite supply of energy showering the Earth on a daily basis. Numerous technologies exist that are directed to capturing the sun's light energy and converting it into electricity. A photovoltaic (PV) module represents such a technology and, to date, has found many applications in areas such as remote power systems, space vehicles and consumer products, such as wireless devices.
A photovoltaic module, or device, functions because of the photoelectric effect. The photoelectric effect of PV devices can be realized by the utilization of semiconducting materials such as silicon (Si), gallium arsenide (GaAs), cadmium sulfide (CdS), cadmium telluride (CdTe), copper indium diselenide (CuInSe2, also referred to as CIS) and copper indium gallium diselenide (CuInGaSe2, also referred to as CIGS). Of these materials, silicon is most frequently used in photovoltaic devices because of: 1) its availability; and 2) its lower cost as compared to the materials GaAs, CdS, CdTe, CIS and CIGS. However, to date, silicon based PV devices have been found to be less efficient than those based on GaAs, CdS, CdTe, CIS and CIGS.
PV modules are known to incorporate PV substrates, such as glass, coated with thin films. Thin film photovoltaics further incorporate a transparent front conductor, usually also a thin film. The most common conductive thin films used are transparent conductive oxides (TCO) such as tin oxide, fluorine-doped tin oxide (FTO), aluminum-doped zinc oxide (AZO) and indium tin oxide (ITO). The main function of a TCO is two-fold. First, the TCO allows light to pass through to an active light absorbing material beneath it. Second, the TCO serves as an ohmic contact to transport photo-generated charges away from the light absorbing material. Such TCOs are desirable for all types of photovoltaic and solar modules, and are especially desirable for photovoltaic modules based on silicon.
Photovoltaic thin films on glass are desirable for numerous reasons. Glass is omnipresent and, as such, provides an existing infrastructure for deployment of PV thin films. Additionally, glass production methods are well known. One such well known glass production method is the float-line method for producing float, or flat, glass. As a result of this desirability for thin films on glass, many methods exist for producing thin film coatings on glass. One of these existing methods is known as “on-line” deposition, wherein a coating apparatus is disposed either in a tin bath of a float-line or downstream of a tin bath of a float-line.
Typically, PV module manufacturers purchase PV substrates that include, for example, the generic structure: glass-substrate/undercoating (UC)/TCO. More specifically, a glass substrate with an undercoating of silicon oxycarbide and a TCO layer of fluorine doped tin oxide, wherein both the undercoating and the TCO layers are deposited pyrolytically in an on-line process.
After obtaining PV module substrates such as those described in the preceding paragraph, a myriad of processing steps must be undertaken to realize the final PV module. Process steps needed for deposition of the semiconducting thin films include, but are not limited to: A) cleaning and washing of the PV module substrate prior to deposition of semi-conductor thin film layers; B) re-heating and re-cooling of the PV module substrate prior to deposition of semiconductor thin film layers; and C) deposition of semiconductor thin film layers. After deposition of the semiconducting thin film layers, further processing steps are required to arrive at the final PV module. These steps include, but are not limited to: D) laser scribing of the silicon layers to form individual PV cells; E) forming a back contact; F) laminating the PV module; G) wiring of the PV module; H) potting of the PV module; and I) testing of the PV module.
The process steps described above with respect to the deposition of the semiconducting thin films impart large amounts of production time and cost associated with the production of PV modules. The amount of time and cost required by the semiconductor thin film deposition steps is one of the major obstacles that hinder electricity generated from PV modules from being economically competitive with electricity generated from fossil fuels. To date, it costs well over $3/peak-Watt (pW) for electricity generated from silicon thin film based PV modules.
While photovoltaics have found many uses, there still exists a number of obstacles to overcome before electricity generated from PV modules can be competitive with electricity generated from traditional fossil fuels. Along these lines, PV module manufacturing costs represent the biggest obstacle preventing electricity generated from PV modules from being competitive with electricity generated from traditional fossil fuels.
Thus, there remains a need in the art for PV module production methods that can overcome the above-noted problems of manufacturing PV modules. In particular, there is a need in the art for PV modules that can be manufactured in a more cost-efficient manner.